1. Field of the Invention
The present invention generally relates to the method of low cost production of aluminum metal matrix nanocomposites. More particularly, the present invention relates to the method of manufacturing nanoscale aluminum oxide particulate reinforced aluminum.
2. Description of the Prior Art
Aluminum Metal Matrix Nanocomposites reinforced by nanoscale aluminum oxide particulates (Al-MMNC) is one type of nanocomposite. A conventional particulate reinforced Aluminum Metal Matrix Composite (Al-MMC) usually has an aluminum or aluminum alloy phase and a ceramic reinforcement phase composed of micron scale particles. Most Al-MMCs material systems in use today are reinforced with aluminum oxide (Al2O3), boron carbide (B4C) and silicon carbide (SiC) particles. Ingot metallurgy (I/M) and powder metallurgy (P/M) are common methods to produce Al-MMCs. Al-MMCs have higher yield strength, stiffness and lower coefficient of thermal expansion than those of the monolithic matrix alloy.
In general, the properties of Al-MMCs are controlled by the particle size, particle distribution in the matrix and volume fraction of the reinforcement as well as the matrix alloy properties. It is found that an Al-MMC reinforced with ceramic particles possessing fine particle size have improved strength, fatigue resistance and machineability composed of Al-MMCs reinforced with coarse ceramic particles.
Optimum mechanical properties of an Al-MMC can be obtained when stable nano ceramic particulate is uniformly distributed in the aluminum matrix. This new composite is considered to be an aluminum metal matrix nanocomposite (Al-MMNC). The I/M Al-MMC manufacturing process can produce a MMC with a ceramic particle size of minimum about 20 microns.
Finer particles are different to wet and distribute uniformly in the metal matrix resulting in microporosity and poor fatigue and fracture toughness. The P/M Al-MMC process can deal with finer particles, but ceramic particles finer than 1 micron are difficult to be distributed uniformly during the mixing process resulting in particle agglomeration.
To create a uniform distribution of the nano ceramic particles, the aluminum matrix alloy powder should have a particle size close to that of the nano ceramic particle. Otherwise, the large aluminum particles are surrounded by very fine ceramic particles. Additionally, the very fine ceramic particles will also agglomerate. These agglomerations are extremely difficult to break-up during blending once they are formed and subsequent thermo-mechanical work of the resultant MMC will cause the agglomerates form “stringers” that will act as crack initiation sites in the material.
Creating a homogenous mixture of two different nano powders or one nano powder with one micron powder is extremely difficult.
High-energy ball mills as disclosed in U.S. Pat. Nos. 4,557,893 and 4,623,388 to Jatkar et al, U.S. Pat. No. 4,946,500 to Zendalis et al and U.S. Pat. No. 4,722,751 to Akechi can mix micron and submicron powders uniformly. They can also mix nano powders uniformly.
However, high-energy ball mills have very low efficiency and are very difficult to scale up for large volume production. This blending technique adds a high cost in addition to expensive nano metal powders and nano ceramic particulates.
To overcome the barrier of mixing two powders using conventional MMC processing methods, an in-situ method has been created. The reinforcing particles are synthesized in the metal matrix by chemical reactions between elements or between the element and compound during fabrication of the MMC.
A variety of in-situ processing techniques for producing a wide range of MMNCs have been developed. X. C. Tong and H. S. Fang, Metall, Mater. Trans. A, 29, 893 (1998), described using an in situ method to produce TiC particulate reinforced Al-MMNC. The TiC particles formed in-situ have a size of about 100 nanometers (nm) and are uniformly distributed in the aluminum matrix.
This processing requires a high cooling rate to reduce the particle size of reinforcement and to modify its distribution in the matrix. It is a major challenge to create uniformly distributed nano reinforcements via chemical reactions during MMNC production without difficult to maintain process control.
There have been no in-situ processes that can produce Al-MMCs reinforced with Al2O3, B4C or SiC, whether on a micronscale or nanoscale.